Drugs currently approved for AIDS therapy belong to classes of compounds that directly attack viral proteins, such as reverse transcriptase (e.g., AZT or zidovudine) or HIV protease. Such compounds have the problem of rapidly developing drug resistance due to rapid transformation of the infecting viral genome. However, despite the problem of rapid drug resistance, such drugs have been used to treat HIV seropositive individuals to help prevent the spread and progression of the infection into the clinical symptoms of AIDS. This problem is exemplified by a recent clinical study (Aboulker et al., Lancet 341:889-890, 1993) showing that AZT treatment of asymptomatic patients in early stages of HIV infection showed no delay in the progression of AIDS disease, although a consistently higher number of CD4.sup.+ T cells was observed in the treatment group when compared to the placebo group, suggesting that CD4 may not be a good marker for AIDS drug efficacy and the need for other approaches for the treatment of AIDS.
HIV infection leads to progressive depletion of CD4+ T lymphocytes and eventual development of clinical symptoms associated with AIDS. After the primary HIV infection, there is a prolonged incubation period of clinical latency that can last as long as twelve years (Weiss, Science 260:1273-1279, 1993). Recent studies indicate viral replication occurs continuously (Pantaleo et al., Nature 362:355-358, 1993) and multiple components of the immune system are chronically activated (Fauci, Science 262:1011-1018, 1993; Bass et al., Clin Immunol Immunopathol. 64:63-70, 1992) during this asymptomatic phase of disease. Persistent activation of the immune system may lead to overproduction of a number of cytokines (Fauci, Science 262:1011-1018, 1993), induction of HIV expression in latently infected cells (Rosenberg et al., AIDS Res. Hum. Retroviruses 5:1-4, 1989), and apoptosis of T cells (Gougeon, Science 260:1269-1270, 1993). Like other retroviruses, all the genes of HIV are expressed under the control of the long terminal repeat (LTR) promoter through multiply spliced mRNAs and precursor proteins that are processed into individual products (Zeichner, Clin. Perinatol. 21:39-73, 1994). Unique DNA and RNA elements in the HIV LTR make this region a target for regulation by many cellular transcription factors as well as the HIV trans-activator protein tat (Gaynor, AIDS 6:347-363, 1992; and Jones et al., Annu. Rev. Biochem. 63:717-743, 1994). The LTR promoter contains cis-acting repressive sequences that inhibit HIV transcription initiation in resting T cells (Lu et al., J. Virol. 64:5226-5229, 1990). Mitogenic stimulation of T cells can activate the LTR promoter to synthesize large amounts of viral RNA. Cytokines, such as tumor necrosis factor (TNF-.alpha.) and interleukin-6 (IL-6), are known to induce HIV synthesis in infected cells (Poli et al., Semin Immunol. 5:165-173, 1993).
HIV infection leads to progressive depletion of specific populations of T lymphocytes and progression of the infection to clinical symptoms associated with AIDS (acquired immunodeficiency syndrome), including HIV-related dementia. Currently, there is no effective treatment for HIV-related dementia, although current therapy, AZT, may have some minor short-term effectiveness.
Microglial cells and TNF.alpha. production have been implicated in the neuropathogenesis of HIV-related dementia. Moreover, microglia and TNF.alpha. release have been suggested to play a role in other neurodegenerative disorders, such as Alzheimer's disease and multiple sclerosis.
The use of immunosuppressive and immunomodulatory agents have been shown to suppress viral replication. Specifically, immunomodulating CD8 lymphocytes have been shown to suppress replication of HIV in peripheral blood mononuclear cells (Walker et al., Science 234:1563, 1986) and activated CD8+ T cells have been shown to inhibit replication of HIV in cultures of CD4+ cells from asymptomatic HIV seropositive individuals (Brinchmann et al., J. Immunol. 144:2691, 1990). Further, the immunosuppressive compound, cyclosporin A (CyA) has a protective effect in several animal models of viral infection. Specifically, chronic treatment with CyA before and after infection with LP-BM5 murine leukemia virus was effective against the development of immunodeficiency disease (Cerny et al., Eur. J. Immunol. 21:1747, 1991). There is also evidence that CyA increases T4 cells and inhibits lymphadenopathy in AIDS and HIV-seropositive, non-AIDS patients (Andrieu et al., Immunol. Immunopath. 46:181, 1988).
The HIV genome encodes at least seven groups of viral proteins (Haseltine, FASEB 5:2349, 1991). These groups of proteins include the three classes of polypeptides present in the majority of animal retroviruses: (1) structural, nonenvelope polypeptides encoded by the gag gene; (2) enzymes required for virion replication (reverse transcriptase/rt) and for cleavage of viral precursor proteins (protease/pr), both encoded by the pol gene; and (3) envelope polypeptides encoded by the env genes. In addition, HIV's possess genes that encode four sets of polypeptides not found in the majority of typical retroviruses. These include a transactivating regulator of RNA translation (tat), a cis-acting downregulator of RNA transcription (nef), a regulator that modulates the expression of structural proteins (rev), a protein that modulates viral infectivity (vif), and two proteins (vpr, vpu) whose functions are still unclear. The genomic order of viral genes encoding the initial portion of each of these proteins (5' to 3') is gag-pr-pol-vif-tat-rev-env-nef.
There are a number of models for predicting the effectiveness of a particular therapy for preventing the progression of HIV infection into an AIDS syndrome and related HIV-related dementia. One such key indicator is the tat protein of HIV type 1 (HIV-1), which is a potent trans-activator of expression of genes from the viral long term repeat (LTR) in vitro, and is essential for viral replication and virus-mediated cytopathicity (Varmus, Genes Dev. 2:1055, 1988 and Cullen, FASEB J. 5:2361, 1991). Tat appears to exert its effect through novel mechanisms that depend upon the recognition of specific, structured, cis-acting viral DNA sequences.
The tat gene encodes a 14 kD protein that transactivates and increases transcription of DNA in HIV-infected cells. The tat gene consists of two separate segments that have been mapped to a location in the HIV genome between the vif and nef regions. It encodes for a 14 kD protein that exists as a metal-linked dimer in infected cells. Products of the tat gene have been shown to greatly augment the rate of viral protein synthesis in HIV-infected cells, and thus to increase the production of HIV virions.
Viral replication of a large number of classes of viruses occurs in a host cell and is often accelerated by primary inflammatory mediators, such as tumor necrosis factor (TNF) (Poli et al., Proc Natl. Acad. Sci. USA 87:782, 1990). Therefore, if a drug were able to ablate or significantly diminish the signal to replicate in a virally infected host cell, such a drug could conceivably block the progression of a virus-based disease in a large number of indications. The invention was made in an effort to find such a drug that acts by such a mechanism of action to have broad spectrum antiviral activity directed toward blocking propagation of the virus but not directly cytotoxic to the virus.
Therefore, there is also a need in the art to develop therapeutic compounds for HIV infection that can block the progression of the disease by acting intracellularly to prevent formation of the tat protein and thereby prevent virion assembly within an infected cell. This invention follows a discovery concerning a functional class of compounds that inhibit a specific group of phospholipid-based second messenger signal amplification pathways also inhibit transactivation of HIV-LTR promoter by tat protein with minimal cytotoxic effects.